Accordingly to a ZigBee network topology, each node in a wireless sensor network system is divided into a ZigBee coordinator (ZC), a ZigBee Router (ZR), and a ZigBee end device (ZB).
The ZigBee coordinator manages an entire tree as a device located at the highest level of a tree structure and manages an entire tree. The ZigBee router is located as a low-level node of the ZigBee coordinator or a low-level node of another router, and communicate by performing synchronization based on a beacon transmitted from the ZigBee coordinator and a high-level router. The ZigBee router may have a low-level node.
As a device which is located at the lowest level on the network topology, the ZigBee end device has a sensor, senses an environment through a sensor, synchronizes the sensed data based on the beacon transmitted from the ZigBee router and the ZigBee coordinator, and transmits the data.
Generally, in a network adopting a ZigBee standard of a tree structure, data sensed and reported by a network ZigBee end device is concentrated in the ZigBee coordinator of the network or routers of small routing depth.
That is, the data sensed and transmitted by the ZigBee end device are not transmitted to routers of large routing depth a lot but are concentrated in the routers of small routing depth, which are located at a high-level of the tree structure.
It will be described in de-tail with reference to FIG. 1 hereinafter.
FIG. 1 is a diagram showing a tree routing method in a beacon mode adopting a conventional ZigBee standard and shows a routing method in a tree structure where a routing parameter is that Lm(Routing Depth)=5, Rm(Max Number of Router)=5, and Cm(Max Number of Child)=7.
Referring to FIG. 1, a router having 4 routing depth (Lm) and ‘1097’ address receives data from 3 devices having ‘1098’, ‘1099’, and ‘1100’ addresses and transmits the received data to a router having ‘1096’ address.
A router having 4 Lm and ‘1105’ address receives data from 4 devices having ‘1106’, ‘1107’, ‘1108’, and ‘1109’ addresses and transmits the received data to the router having ‘1096’ address.
Meanwhile, since a router having 3 Lm and ‘1096’ address receives data from own child routers having ‘1097’ and ‘1105’ addresses and transmits the data to a parent router having ‘1095’ address, the router should transmit/receive the larger number of data than the child routers having ‘1097’ and ‘1105’ addresses.
In consideration of data transmission/reception according to the routing depth (Lm), the network ZigBee coordinator should transmit/receive the larger number of data than the router which is located in a network end.
FIG. 2 shows a superframe allocated in a beacon mode adopting a conventional ZigBee standard.
Referring to FIG. 2, all ZigBee coordinators and ZigBee routers on the same network based on ZigBee have the same superframe length which is expressed as a Superframe Order (SO) value with no regard to the routing depth.
When the routing depth (Lm) value of the routing parameter is small, when the number of devices installed on the network is small, or when the data are not generated by an event in the devices, there is no problem in operation. However, when the routing depth (Lm) value of the routing parameter is large or when the sensor network has the large number of devices installed on the network, data are concentrated in the ZigBee coordinator or routers having small routing depth. Accordingly, the data are not normally transmitted, thereby causing problems such as losing of data or exceeding of data transmission time.
The superframe length is extended in order to solve the above problem as shown in FIG. 3.
FIG. 3 is a diagram showing a superframe whose length is extended in a beacon mode adopting a conventional ZigBee standard and shows a superframe allocated to transmit/receive the large number of data.
Referring to FIG. 3, when the superframe length of all ZigBee coordinators and ZigBee routers on the network is identically extended, the ZigBee coordinator and the routers having small routing depth can safely operate.
However, when all ZigBee coordinators and ZigBee routers have a large superframe order value for the ZigBee coordinator and the ZigBee routers having small routing depth, routers which do not require the large number of data communication, i.e., routers having a large routing depth, have the same superframe order value and a problem that frequency resources are wasted is generated.
Also, when all ZigBee coordinators and ZigBee routers have a large superframe order value, there is a problem that the number of routers which can be accepted in the same beacon interval is limited. That is, since a beacon order value for determining the beacon interval is the same, there is a problem that only a comparatively small number of routers can access to the network.